The present invention relates to a photosensitive lithographic printing plate, and in particular to such that is suited for imaging with a laser light.
Conventionally, as the photosensitive lithographic printing plate, there have been widely used so-called PS plates comprising a hydrophilic support and an oleophilic photosensitive resin layer provided on the support. Plate making with such a plate has usually been carried out by a masked (planar) exposure with use of a lith film, followed by elimination of non-image areas by dissolution.
In recent years, digital technologies with which image information is electronically processed, stored and outputted with a computer is prevailing. To come with such digital technologies, a variety of image outputting processes have been developed for practical use. As a result, the computer-to-plate (CTP) technology that modulates a highly directional light such as a laser beam in accordance with digitized image information for scanning exposure to directly prepare a printing plate without using a lith film is eagerly demanded. Hence, the development of a photosensitive lithographic printing plate adapted to the CTP technology is a significant technical target.
For such photosensitive lithographic printing plates suited for scanning exposure, a structure comprising a hydrophilic support and provided thereon an oleophilic photosensitive resin layer (which will be referred to as a photosensitive layer hereinafter) capable of generating an active species such as a radical or Bronsted acid by laser exposure has been proposed, and is already available in the market. A negative or positive type lithographic printing plate can be obtained by exposing such a photosensitive lithographic printing plate to a scanning laser light modulated by digital information to generate such active species, causing a physical or chemical change in the photosensitive layer to make the layer insoluble or soluble by the action of the species, and subjecting the plate to a subsequent development processing.
In particular, the negative type photosensitive lithographic printing plate comprising a hydrophilic support and provided thereon a photo-polymerizable photosensitive layer containing a photo polymerization initiator excelling in photographic speed, an ethylenically unsaturated compound capable of addition polymerization, a polymer binder soluble in an alkaline developer, and an oxygen-shielding protective layer which may be provided according to need is endowed with a potential of becoming a plate having desirable printing characteristics due to its various advantageous features including an excellent productivity, simple processing, and a high resolution as well as a superior ink-receptivity.
Conventionally, in order to make a printing plate durable for long run lengths, urethane resins have been used as the binder as described in Japanese Patent Publication Nos. 120040/1995, 120041/1995, 120042/1995and 12424/1996, Japanese Patent Laid-Open Nos. 287944/1988, 287947/1988 and 271741/1989, etc. However, these patent specifications include no description on the imaging with laser scanning exposure at all. On the other hand, Japanese Patent Application No. 116232/1999 describes a lithographic printing plate comprising a urethane binder suited for laser scanning exposure, but this type of lithographic printing plate is not satisfactory in printing durability and productivity. Specifically, when the scanning speed is raised for a higher productivity, the amount of exposure energy per unit area correspondingly reduces to cause the discrimination between the exposed and the unexposed areas to reduce whereby the image area tends to be attacked by the alkaline ingredient in the developer, leading to insufficient printing durability.
As a polymer binder constituting a photosensitive layer there has also been used an alkali-developable organic high molecular polymer such as methacrylic acid copolymer, acrylic acid copolymer, itaconic acid copolymer, crotonic acid copolymer, maleic acid copolymer and partially esterified maleic acid copolymer as described in Japanese Patent Laid-Open No. 1984-44615, Japanese Patent Publication No. 1979-34327, Japanese Patent Publication No. 1383-12577, Japanese Patent Publication No. 1979-25957, Japanese Patent Laid-Open No. 1979-92-723, Japanese Patent Laid-Open No. 1984-53836, Japanese Patent Laid-Open No. 1984-71048, etc.
However, the conventional negative-working photosensitive lithographic printing plate comprising a photosensitive layer containing such a polymer binder is disadvantageous in that when the scanning speed is raised for a higher productivity, the amount of exposure energy per unit area correspondingly reduces, making it impossible to effect sufficient hardening even at the exposed area and hence causing the image area to be attacked by the alkaline ingredient in the developer and leading to insufficient printing durability. Thus, further improvement of productivity is made difficult.
The object of the invention is to provide a photosensitive lithographic printing plate which overcomes the above-mentioned shortcomings in the conventional technologies, is capable of forming a highly durable printing plate, suited for scanning exposure with laser, capable of high speed image recording, and provided with a high productivity.
As a result of an extensive investigation, the present inventors have found that the object cited above can be achieved by using, as the polymer binder for the photosensitive layer, a urethane resin which has an aliphatic cyclic structure having a carboxylic group as a substituent directly or indirectly attached to said structure as the component imparting a developability to said urethane resin.
Accordingly, the invention provides, as a first preferred embodiment, a photosensitive lithographic printing plate comprising a support and a photosensitive layer provided thereon, said photosensitive layer comprising a polyurethane resin binder having an aliphatic cyclic structure which has a carboxyl group as a substituent directly or indirectly attached to said structure.
In the first preferred embodiment, a feature of the photosensitive lithographic printing plate of the invention is to use such a urethane resin as a polymer binder as comprising an alicyclic structure having a carboxyl group directly or indirectly attached thereto as a substituent imparting an alkali-developability to said urethane resin. By the use of such a binder, the photosensitive lithographic printing plate of the invention is imparted with a sufficient printing durability as well as a high productivity even with exposure conditions of low irradiation energy per unit area.
Although the mechanism with which the advantageous feature of the invention is developed is not clarified yet, it is assumed that the introduction of a highly water-repellent cyclic hydrocarbon structure near the carboxylic acid group prevents the penetration of the developer, thus giving rise to a high printing durability, in contrast to the conventionally known urethane binders whereby the developer penetrates into the image area obtained by an exposure of a relatively small irradiation energy. Due to such a mechanism, printing plates with both of a high productivity and a high printing durability can be obtained.
The inventors made extensive studies of accomplishment of the foregoing object. As a result, it was found that the foregoing object of the invention can be accomplished by using as the polymer binder constituting the photosensitive layer a modified polyvinyl alcohol resin containing an alicyclic structure. The present invention has thus been worked out.
The present invention provides, as a second preferred embodiment, a photosensitive lithographic printing plate comprising a photosensitive layer containing a polyvinyl alcohol resin binder modified with an acetal skeleton having an alicyclic structure.
In the second preferred embodiment, a feature of the photosensitive lithographic printing plate of the invention is to use as a polymer binder a polyvinyl alcohol resin modified with an acetal skeleton having an alicyclic structure. In this arrangement, the photosensitive lithographic printing plate of the invention is imparted with a sufficient printing durability as well as a high productivity even with exposure conditions of low irradiation energy per unit area.
Although the mechanism with which the advantageous feature of the invention is developed is not clarified yet, it is assumed that the use of a polyvinyl alcohol resin modified with an acetal skeleton having a highly hydrophobic alicyclic structure prevents the penetration of the developer, thus giving rise to a high printing durability, in contrast to the conventionally known polymer binder whereby the developer penetrates into the image area obtained by an exposure of a relatively small irradiation energy. Due to such a mechanism, printing plates with both of a high productivity and a high printing durability can be obtained.